Biomass has been the primary source of energy over most of human history. During the 1800's and 1900's the proportion of the world's energy sourced from biomass dropped sharply, as the economical development of fossil fuels occurred, and markets for coal and petroleum products took over. Nevertheless, some 15% of the world's energy continues to be sourced from biomass, and in the developing world, the contribution of biomass to the energy supply is close to 38%.
Solid biomass, typically wood and wood residues, is converted to useful products, e.g., fuels or chemicals, by the application of heat. The most common example of thermal conversion is combustion, where air is added and the entire biomass feed material is burned to give hot combustion gases for the production of heat and steam. A second example is gasification, where a small portion of the biomass feedstock is combusted with air in order to convert the rest of the biomass into a combustible fuel gas. The combustible gas, known as producer gas, behaves like natural gas but typically has between 10 and 30% of the energy content of natural gas. A final example of thermal conversion is pyrolysis where the solid biomass is converted to liquid and char, along with a gaseous by-product, essentially in the absence of air.
In a generic sense, pyrolysis or thermal cracking is the conversion of biomass, fossil fuels and other carbonaceous feedstocks to a liquid and/or char by the action of heat, normally without using direct combustion in a conversion unit. A small quantity of combustible gas is also a typical by-product. Historically, pyrolysis was a relatively slow process where the resulting liquid product was a viscous tar and “pyrolygneous” liquor. Conventional slow pyrolysis has typically taken place at temperatures below 400° C. and at processing times ranging from several seconds to minutes prior to the unit operations of condensing the product vapors into a liquid product. The processing times can be measured in hours for some slow pyrolysis processes used for charcoal production. The distribution of the three main products from slow pyrolysis of wood on a weight basis is approximately 30-33% liquid, 33-35% char and 33-35% gas.
A more modern form of pyrolysis, termed fast pyrolysis, was discovered in the late 1970's when researchers noted that an extremely high yield of a relatively non-viscous liquid (i.e., a liquid that readily flows at room temperature) was possible from biomass. In fact, liquid yields approaching 80% of the weight of the input woody biomass material were possible if the pyrolysis temperatures were moderately raised and the conversion was allowed to take place over a very short time period, typically less than 5 seconds. In general, the two primary processing requirements to meet the conditions for fast pyrolysis are very high heat flux to the biomass with a corresponding high heating rate of the biomass material, and short conversion times followed by rapid quenching of the product vapor. Under the conditions of fast pyrolysis of wood the yields of the three main products are approximately, 70-75% liquid, 12-14% char, and 12-14% gas. The homogeneous liquid product from fast pyrolysis, which has the appearance of espresso coffee, has since become known as bio-oil. Bio-oil is suitable as a fuel for clean, controlled combustion in boilers, and for use in diesel and stationary turbines. This is in stark contrast to slow pyrolysis, which produces a thick, low quality, two-phase tar-aqueous mixture in very low yields.
In practice, the fast pyrolysis of solid biomass causes the major part of its solid organic material to be instantaneously transformed into a vapor phase. This vapor phase contains both non-condensable gases (including methane, hydrogen, carbon monoxide, carbon dioxide and olefins) and condensable vapors. It is the condensable vapors that, when condensed, constitute the final liquid bio-oil product, and the yield and value of this bio-oil product is a strong function of the method and efficiency of the downstream capture and recovery system. The condensable vapors produced during fast pyrolysis will continue to react as long as they remain at elevated temperatures in the vapor phase, and therefore must be quickly cooled or “quenched” in the downstream process. If the desired vapor products are not rapidly quenched shortly after being produced, some of the constituents will crack to form smaller molecular weight fragments such as non-condensable gaseous products and solid char, while others will recombine or polymerize into undesirable high-molecular weight viscous materials and semi-solids.
As a general rule, the vapor-phase constituents will continue to react at an appreciable rate, and thermal degradation will be evident, at temperatures above 400° C. If a fast pyrolysis process is to be commercially viable, it is therefore extremely important to instantaneously quench the vapor stream, after a suitable reaction time, to a temperature below about 400° C. preferably less than 200° C. and more preferably less than 50° C. Such a requirement to rapidly cool a hot vapor stream is not easily accomplished in scaled-up commercial fast pyrolysis systems. As the rapid cooling is effected, certain components in the vapor stream (particularly the heavier fractions) tend to quickly condense on cooler surfaces (i.e., transfer lines and ducting to the condensers) causing deposition and fouling of the equipment, and also resulting in the creation of a mass of warm liquid where additional secondary polymerization and thermal degradation can occur. In these regions where there is a temperature gradient between the hot reaction temperature and the lower condenser temperature, it is therefore critical to mitigate against condensing vapor deposition and the occurrence of resultant unwanted thermal reactions. The condensation and deposition phenomena described above can also apply to the thermal conversion of petroleum, fossil fuel and other carbonaceous feedstocks (e.g., the thermal upgrading of heavy oil and bitumen).
Therefore, there is a need for systems and methods that reduce such deposition and mitigate secondary reactions.